This invention relates generally to devices for measuring eyes and more particularly to an ophthalmic measuring instrument which can be applied directly to an eyeglass frame selected by the patient so that a monocular measurement of the patient's visual axis can be obtained for each respective eye, which measurements are critical in prescribing lenses, telescopes, and microscopes for patients with special optical needs.
Certain words of art are utilized in the present application and when used herein, these words are defined as follows:
a. Geometric axis of the eye is the imaginary line passing through the anterior and posterior poles of the given eyeball. PA0 b. Pupillary axis is the imaginary line perpendicular to the corner and passing through the center of the entrance pupil of the eye. PA0 c. Visual axis is the imaginary line connecting the fovea (the point on the retina which fixates on a target) to the point of fixation on the target and passing through the nodal points of the eye. PA0 d. PD is the shortened term for pupillary distance which is the distance between the respective pupillary axis of each eye.
For the normal eye, the pupillary axis closely parallels the geometric axis. The PD or pupillary distance is a topographical measurement that is used to grossly determine the direction in which the eyes are gazing.
The visual axis however is a psychophysical imaginary line which precisely connects the fovea and the target being viewed.
When formulating the prescription for corrective lenses, the conventional technique for determining the geometric centers for such corrective lenses is to measure the pupillary distance (PD) for the patient's eyes. This has been done by the examiner's use of a metric ruler or other device for measuring objectively the distance between the inner edge of one pupil and the outer edge of the other pupil of the patient's eyes as the eyes are fixed on a far point target and then on a near point target.
Because of the importance of these measurements to the examiner in determining the proper prescription for the patient's lenses particularly in low vision patients, various devices are known in the prior art for making accurate objective measurements of the PD distance for a patient's eyes such as is shown in U.S. Pat. Nos. 1,052,161, and 2,197,139, and 2,491,312 and at least U.S. Pat. No. 2,884,832 outlines a method for patient or subjective measurement of PD.
In U.S. Pat. No. 2,491,312 the problems and the reasons for such measurements particularly in the filling and providing of bi-focal and multi-focal lenses are set forth in some detail.
It has been found that scrupulously derived results from the conventional twenty-one point optometric examination can be voided by inaccurate PD measurements for the far point and near point working distances of the patient.
Some prior art methods of measurement may be adequate to provide PD measurements for distance vision excepting eccentric fixation. However, the use thereof for providing the PD measurement for near vision often results in errors which can cause induced prism and cylinder, in turn causing asthenopic and even aniseikonic symptoms. Prentice's rule known to those skilled in the art has shown that prism is a function of dioptric power times centemeters of displacement. Therefore, any error in these conventional methods of objective measurement of PD become critical when prescribing a high dioptric value prescription.
While in the normal eye, the pupillary axis closely parallels the geometric axis, one critical source of error which the conventional methods of objective measurement of PD distance for a patient's eyes does not and cannot account for is the known fact that the visual axis for the eye passes 1 mm. nasal to the geometric axis for the given eye. This nasal characteristic of the visual axis is not significant for distance vision because the visual axes are essentially parallel and therefore the conventional methods of objective measurement for the PD of the patient's eyes and the subjective or patient's measurements of the visual axes for the eyes will be essentially in agreement except for eccentric fixation.
When, however, these methods of measurements are applied to near vision, the inter-visual axes distance will be consistently smaller because of the convergence of the visual axes at the vertex distance of the plane of the eyeglass frame.
Such inaccuracies of the conventional objective measurement techniques can be tolerated for most sphero-cylindrical refractive errors. This is not the case for spherical refractive errors above 4.00 diopters (D) and cylinder above 1.00D and/or where a patient requires high refractive lenses, telescopes and/or microscopes. If these inaccuracies and errors are present they can cause many vision problems.
The monocular measurement of the patient's visual axis cannot be overlooked for the further reason that optical and facial assymetries can cause the geometric center of the lens for each eye to be displaced from the point at which the visual axes actually passes through the lens. Facial assymetry on each patient is such that the distance between each eye and the bridge of the nose is seldom equal.
In U.S. Pat. No. 2,884,832, a device and method is disclosed for measuring the inter-pupillary axes distance for a given patient's eyes by measuring the monocular pupillary axis distance for each of the respective eyes of the patient and the near convergence of the pupillary axes by measuring the pupillary distance (PD) for the combined or binocular action of both eyes. However, the device utilized with this method of measuring can introduce error when used for locating the position of the geometric or optical center in a lens prescription because the pupillary axis for a given eye does not intersect the plane of the lens at the same location as the visual axis.
The subjective use of a relatively wide filter strip in front of the patient's eye as described in the method of U.S. Pat. No. 2,884,832 and in the related article in the Optemetric Weekley 52:39 of Sept. 28, 1961 by the inventor, O.R. Engleman entitled--Subjective Pupillary Distance Measurement--permits the patient to project off-foveal or peripheral retina points to the pupillary margins and through the filter strips. This results in an extension of the pupillary angle of which the line will theoretically converge behind the eye. The line bisecting this angle is the pupillary axis.
Since the average pupil size is approximately 4 mm., using filter strips relatively larger than the pupil allows the pupillary angle and pupillary axis to be accurately measured by this method. However, this measurement can be grossly in error for prescription purposes because of the problem set forth above.
Further error is introduced by the method and device shown and described in U.S. Pat. No. 2,884,832 and the above-mentioned related article because it is difficult for the patient and the examiner to set and to maintain with the setting element thereof the vertical position of the visual axis for a given eye being measured, it does not allow vertical or horizontal measurement of the inclination of the visual axis which is important when prescribing spectacle-mounted telescopes and surgical binoculars, and the width of the target card used does not equal the visual angle subtended through the setting or viewing means of the device at the working distances.
The present invention seeks to meet and overcome these problems by providing a new device which permits the examiner to define the visual axis for each of the patient's eyes. This device is affixed to the eyeglass frame selected by the patient so that the center of the eyeglass frame serves as a reference point. On this device, adjustable relative narrow filter strips having a width in a range from 2 to 3 11/2 mm. are mounted in front of each eye viewing window on the eyeglass frame for movement relative the associated eye viewing window and for intersecting relation to each other. By placing the eyeglass frame with the improved device in accordance with the present invention thereon, in position to the patient's face, and having the patient view sized targets for the near working distance therethrough, the patient can subjectively provide the examiner with measurements of; the monocular visual axis (MVA) distance of the respective eyes of the patient relative the center of the eyeglass frame; measurements of the exact horizontal and vertical displacement of the visual axes for the respective eyes of the patient, so as to eliminate all possible sources of error and provide a relatively simple standardized method for measuring the inter-visual axes for a patient's eyes from which an examiner can more accurately prescribe the required corrective lenses, telescopes and microscopes for patients with special optical needs.
Control of the width of the filter strips in the device according to the present invention so that the respective filter strips are narrower than the pupil of the given eye being examined is necessary for accurate measurement of the visual axis. When a filter strip narrower than the pupils is used, the fovea, the point on the retina of the eye is used to sight directly through the filter. The angle projected from the fovea through the filter is the visual angle. The imaginary line bisecting this angle is the visual axis.
Since, even in a normal eye, the visual axes passes through the cornea and the lens of the patient's eyeglasses nasal to the pupillary axis, the difference between the visual axis and the pupillary axis will become more pronounced when the pupil is displaced, defined as corectopea, by either pathology or surgery. Errors on these situations can produce differences between the pupillary axis and the visual axes at the plane of the lens of the eyeglass frame of as much as 6 to 7 mm. and if the conventional measurement of the pupillary axes is utilized, gross errors can be introduced in determining the prescription for the patient's lenses.
Thus, unlike prior art devices and methods such as is shown in U.S. Pat. No. 2,884,832 which measures only PD, the device in accordance with the present invention is measuring a completely different axes, namely the visual axes and provides a method for measuring the inter-visual axis so that the patient's prescription can be determined free of all possible sources of errors and with a precise position for the visual axes at the plane of the respective lenses in the patient's eyeglasses.